1. Field of the Invention
The present invention relates to a method for producing an optimum sound field of loudspeakers, especially to a method for producing an optimum sound field of loudspeakers in which reference points are set around a required sound field to get audio signals and related gradient on the boundary and an optimization method is used to calculate output signals of each loudspeaker for producing a sound field required. Thus the listeners can still get best sounds even they turn their heads or move around.
2. Description of Related Art
The conventional stereophonic recording methods were firstly developed by Blumlein. Later, an integrated and multi-level environment such as Dolby Digital or digital theater system (DTS) surrounding technology is produced by a plurality set of loudspeakers so as to pursue a virtual reality stereo system and a better acoustic environment. Generally, stereophonic sound can be achieved by using at least two audio channels, through a configuration of several loudspeakers, to provide sound heard from various directions, as in natural hearing. In theaters or cinemas, a plurality of loudspeakers is placed around a performance space to surround the audience, allowing the audience to hear sounds coming from all around them. This is wave field synthesis (WFS) technique that creates virtual acoustic environments by a large number of loudspeakers.
Moreover, due to fast progress of technology and higher standard of living, people have higher requirements for electronic devices that display images and sounds. They concern not only the image quality, but also the quality of the sound field. However, the surround loudspeaker distribution formed by placement of a plurality of loudspeakers is of high cost. In a confined or narrow area such as parlors or rooms, the optimal placement of the loudspeakers may not easily be achieved. In 1992, Dolby laboratory has developed surround sound multichannel audio system 5.1 by using digital audio encoding technologies. The 5.1 surround sound system is the most common in home theaters. The 5.1 surround sound speaker system includes five speakers-two left and right front speakers, two left and right rear speakers, a quality center speaker, and a powered subwoofer for deep, rumbling bass tones. Except the subwoofer, other speakers are highly directional so as to create a surround sound field. When users are in such sound field, they have an optimum surround sound experience at home as in the theater. In order to provide optimum sound quality, the placement of the speakers needs to meet certain requirements of angles and distances to the listener. However, these requirements are often at odds with the space constraints such as a limited space in vehicles or a compact room arrangement. Thus the best placement of the speakers is unable to be achieved and users can't get an even, all-surrounding audio experience.
A technique that reproduces optimum stereo sound by small-scale loudspeaker array units has been developed. It's based on the ability of humans to localize sound sources by two binaural cues-interaural time differences of arrival and interaural intensity differences. The time difference is defined as the difference in arrival time of a sound at the left and right ears while the intensity difference is the amplitude difference generated between the right and left ears. A head related transfer function (HRTF) is used to simulate surround sound playback from loudspeakers fixed at certain positions. By adjusting audio frequency and delay of HRTF, stereo sounds are simulated and played by the fixed loudspeakers. The conventional HRTF describes time and amplitude differences of binaural response for a listener. The differences are generated due to anatomical structure of the listener's head and pinnae and are used to detect the position of the sound source. The technique by using binaural time differences and amplitude differences, together with reverberation, chorus to have auditory perception is called duplex theory. Refer to FIG. 4, a schematic drawing showing a sound field produced between two ears of a listener by duplex theory is revealed. But this technique has two shortcomings. One of the limitations of the duplex theory is that it doesn't differentiate sounds coming from the front and the sounds from the rear due to symmetry of the listener's head assumed in the duplex theory. The other is that an optimum sound field synthesized by duplex theory is around two ears of the listener. Once the listener turns the head or shifts the body, the two ears leave the range of the optimum sound field. This affects the auditory perception and even causes the listener having uncomfortable feelings.
Thus there is a room for improvement and a need to provide a novel method for producing a sound field.